Square drain extension



FIG. 1 is a bottom plan view of a square drain extension, showing our new design;

FIG. 2 is a back side elevation view thereof; the front, right and left sides being an identical image;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom perspective view thereof; and,

FIG. 5 is a top perspective view thereof.

The broken lines shown in the drawings depict portions of the square drain extension that form no part of the claimed design. 

CLAIM The ornamental design for a square drain extension, as shown and described. 